The utility of natural and synthetic bi-dimensional substrates (i.e., web materials such as films, woven and non-woven fabrics, paper, etc.) can be enhanced by surface treatment of the substrate to tailor the substrate surface characteristics. For example, a bi-dimensional substrate may be coated to improve the barrier properties of the substrate material. The surface energy of a bi-dimensional substrate may be altered to improve wettability for improved printing, gluing, or coating of the substrate. The electrical polarity of a bi-dimensional substrate may be altered to improve the adhesivity of the material. The surface resistance of a substrate material may be altered to improve electrostatic properties. The coefficient of friction of a bi-dimensional substrate may be adjusted to improve or reduce slipping of the material. In general, surface treatment may be used to create high value added materials from more economical materials.
A conventional and traditional method for surface modification of bi-dimensional substrates is wet surface modification. By this method, a surface treatment material, such as a coating medium, in liquid form, is applied to the substrate surface. The liquid surface treatment material may be applied to the substrate surface by, for example, spraying or extruding the material onto the substrate surface or dipping the substrate into the treatment material.
Another, and more advanced, method for surface treatment of bi-dimensional substrates is plasma treatment. For plasma treatment, a substrate to be treated is placed in a reaction chamber along with a reactive gaseous material. The reaction chamber may be sealed, and the gas pressure level within the sealed reaction chamber set to a desired pressure level. Electrodes are positioned within the reaction chamber close to the substrate surface to be treated. A high voltage current signal (DC, AC, or high frequency (RF) depending upon the surface treatment material and process employed) is applied to the electrodes. The resulting electron discharge generates a plasma near the surface of the substrate being treated. A potential (such as ground potential) may be applied to or near the substrate material to draw the plasma into the material. Plasma treatment modifies the chemical nature of the surface layer of a substrate material, thereby affecting the surface properties of the material in the desired manner. Plasma treatment may be used to improve the bonding of coatings such as adhesives, dyes, inks, polymers and photographic emulsions to the bi-dimensional substrate, to improve the wettability of hydrophobic materials, or to alter mechanical properties, such as the coefficient of friction or the cohesiveness of textile fibers. In another variation, polymer films may be deposited on most surfaces by plasma polyermization from a discharge in an appropriate gaseous monomer. Apart from their potential use as bonding layers, such thin films find other possible uses, including protective and decorative layers, optical coatings, capacitor dielectrics and semi-permeable membranes.
Most of the research carried out in the field of surface treatment of organic and inorganic substrates under various plasma conditions is based on batch-type processing. Batch-type processing involves loading a substrate to be treated into a vacuum chamber, evacuating the vacuum chamber (typically while admitting a reactive gas from which the plasma will be generated into the vacuum chamber), processing to provide the desired surface treatment by generating a plasma, re-pressurizing the vacuum chamber, and unloading the finished treated substrate. This process must be repeated for each substrate to be treated. It is apparent that batch-type processing requires many steps, is costly, and cannot achieve high-productivity rates. Thus, plasma-based surface treatment by batch-type processing is not technologically or economically feasible for many industrial applications. This is especially true for the surface treatment of continuous filaments, webs, films, and the like. The treatment of such materials is only cost-affective if such substrates can be transported into a reaction chamber, through a plasma zone, and evacuated from the reaction chamber in a continuous manner without affecting the pressure level within the reaction chamber, allowing outside air into the reaction chamber, or allowing gaseous plasma components to escape from the reaction chamber. Various methods and systems have attempted to achieve a continuous flow of bi-dimensional substrate material to be treated through a sealed reaction chamber.
A basic approach for surface treating continuous uni-dimensional or bi-dimensional materials in a plasma chamber is to pass the material to be treated into and out of the chamber through a narrow hole or slot in the chamber. Such so-called slot seals are not very affective. Slot seal systems are characterized by high gas-leak rates into the reaction chamber through the slot seals. As a consequence, high pumping rates are required to maintain a desired pressure level within the chamber. Also, due to the open slot nature of the "slot sealing," significant environmental gas contamination dominates the treatment process.
Another sealing system which has been employed uses liquid seals. In such systems, the continuous bi-dimensional material to be treated is passed through a liquid barrier into and out of the treatment chamber. A liquid seal can provide a more effective seal between the interior of the treatment chamber and the outside environment than a slot seal. However, liquid sealing systems are not popular due to the contamination effects of the sealing liquids. Even if the solid material to be treated and the liquid sealing material are dissimilar, small liquid quantities retained on film and web surfaces through dispersion and capillary forces can induce undesired deposition effects under the discharge environment within the treatment chamber.
Belt sealing systems have also been employed to transport a continuous flow of bi-dimensional material to be treated into and out of a reaction chamber. However, such systems are complex and generally ineffective. Belt sealing systems do not provide sure vacuum sealing, and will not allow low pressure operations to be achieved.
The most promising approaches for sealing a vacuum reaction chamber while transporting a bi-dimensional substrate through a reaction zone within the chamber employ roller seals. Noncompliant and/or compliant rollers, depending on the nature of the substrate, are employed to transport the bi-dimensional substrate into and out of the reaction chamber. A seal is achieved between the roller surfaces and the surfaces of the bi-dimensional substrate at transport contact points between the rollers. Non-compliant rollers alone do not allow proper control of the pressure exerted by the rollers on the substrate. Compliant rollers alone create serious friction problems on roller-housing surfaces. Therefore, a combination of non-compliant and compliant rollers is typically employed.
An exemplary known apparatus for plasma treatment of continuous bi-dimensional substrate material which employs a roller seal system is described in U.S. Pat. No. 5,314,539 to Robert W. Brown, et al. This patent describes a vacuum chamber for the treatment of materials of continuous length (e.g., films for photographic support) using a multiple roller system. In the system described, a single vacuum treatment chamber is divided into multiple sub-chambers. Roller systems separate the sub-chambers from each other, and transport the continuous material through the sub-chambers. For example, three rollers may be placed side-by-side across a vacuum chamber to divide the chamber into two sub-chambers. Outer side rollers in each three roller system have non-compliant surfaces, which are sealed to the vacuum chamber walls via face sealing elements composed of a rigid material resiliently biased into engagement with the outer side roller surfaces. A central roller in the three roller system has a compliant outer surface. A bi-dimensional material is transported from a first sub-chamber to a second sub-chamber between a first outer side roller and the central roller of the three roller system, and back out of the second sub-chamber into the first sub-chamber between the other outer side roller of the three roller system and the central roller. End sealing (sealing between the vacuum chamber walls and the roller end surfaces) is achieved using elongated end-sealing elements. These sealing elements extend across the rollers between the roller end surfaces and a chamber wall. This arrangement assures an uninterrupted sealing surface at the roller ends. The individual sub-chambers created by multiple roller systems positioned within a single vacuum chamber may be separately evacuated. By creating individual vacuum stages, a very low vacuum level can be achieved in at least one of the sub-chambers. Plasma treatment, under DC or RF conditions, can take place in this chamber. A significant limitation of the multi-stage vacuum system described by Brown, et al., and of most other systems employing roller seals, is the complexity of the system and, specifically, the high number of roller seals employed to achieve effective sealing of the reaction chamber. Problems associated with the use of numerous rotating and stationary large sealing surfaces in roller sealing systems limits the ability to employ such systems in industrial settings.